Conventionally, as a measure for achieving improvement in fuel economy of automobiles, weight reduction of various components has been pursued. For example, an aluminum alloy is used instead of cast iron as a material for an engine block, or a magnesium alloy is used instead of steel as a material for an engine cover or an oil pan.
In the case where the components made of the aluminum alloy or magnesium alloy as mentioned above are fastened by a conventional steel bolt, looseness is likely to occur due to a large difference between the linear expansion coefficient of the aluminum alloy or the magnesium alloy and the linear expansion coefficient of the steel, and corrosion due to contact of dissimilar metals is also likely to occur. Therefore, in order to sufficiently secure the reliability of fastening, it is necessary to deepen the screw hole of the component to lengthen the shaft section length of the steel bolt, or thicken the diameter of the steel bolt. However, the length of the shaft section of the steel bolt influences the wall thickness of the component, and meanwhile, the thickness of the steel bolt influences the width of the flange portion providing the screw hole of the component. Thus, the use of the steel bolts for fastening the components made of aluminum alloy or magnesium alloy has been an obstacle in pursuing the weight reduction.
In order to solve such a problem of steel bolts, the movement of adopting an aluminum alloy bolt as a fastening member for fastening the components made of an aluminum alloy or a magnesium alloy has also been expanded (see, for example, Patent Literature 1). In Patent Literature 1, a 6000 series aluminum alloy is disclosed as a material for a bolt which is excellent in workability at the time of manufacturing the bolt and has sufficient strength. Since the aluminum alloy bolts have small difference in linear expansion coefficient with aluminum alloys or magnesium alloys constituting various components and small galvanic corrosion, even if the screw hole of the component is made shallow or the diameter of the bolt is thinned, the reliability of the fastening can be secured, which is suitable for achieving weight reduction.
2000 series aluminum alloy is known as an aluminum alloy having a higher strength than the aforementioned 6000 series aluminum alloy. When using 2000 series aluminum alloy, it is possible to make bolts of higher strength than bolts made of 6000 series aluminum alloy.